[kh99]

I don't know if there's some standard option to do that, but assuming there isn't you could create a plugin using the newthread_start hook with code something like:

$min_posts = 10;
if ($vbulletin->userinfo['posts'] < $min_posts)
{
	eval(standard_error("You must have ".$min_posts." posts before you can start a thread."));
}